Package to board interconnection has been accomplished using many different methods over the years. The industry was initially dominated by pin-through-hole (PTH) lead attachment with “integrated circuit” packages that were rectangular in shape and had rather large leads extending from the long side of the rectangle. These devices were limited in lead count and provided very rugged interconnection between the package and the printed circuit board (PCB). PTH technology was gradually replaced by surface mount technology in order to increase the number of leads, and to improve the automation of the process for attaching the devices to the boards. In the recent years, a new packaging technology, known as “ball grid array” (BGA) technology, has been developed. A BGA package consists of a silicon chip attached to the surface of a substrate. The substrate has printed circuitry that provides interconnect points for the silicon chip on the top surface, connected by fine pitch traces to an array of pads on the bottom surface. The pads on the bottom surface have attached solder spheres that serve as the interconnect points for the package to the PCB. The BGA technology allows designs with lead counts of over 1000 input/output points. In addition to the high lead count, this technology also affords many other benefits that include ease of handling, simplified device attachment and overall cost effectiveness compared to fine pitch, perimeter leaded devices.
The BGA technology, however, has a compromised reliability in thermal cycling. A perimeter leaded device with gull wings can be subjected to many thermal cycles without encountering stresses due to thermal coefficient of expansion (TCE) mismatch between the PCB and the device. BGA packages, on the other hand, are connected to the board with a rigid structure of solder spheres (oval shaped after reflow). When the device is operated, waste heat builds up and a temperature differential between the device and the board is created. The temperature differential, as well as the differences in TCE between the device and the board, will lead to stresses in the solder sphere attachment points, which creates a high risk of fatigue failure.
One solution to the thermal stability problem is the column grid array (CGA) technology, which utilize a flexible column lead in place of the solder spheres. The column leads are designed to have a lower stiffness than a solder sphere and a higher offset distance between the device and the PCB. These two features enable the leads to flex with less stress as the dimensional expansion between the device and the PCB varies. The higher offset distance reduces the stress by the square of the distance between the device and the PCB.
CGA has been widely used in high reliability applications. However, the thin and tall solder column interconnects in CGA are susceptible to damage due to short-term dynamic load during shock, vibration, and creep under long-term static compressive load. For example, a thermal solution that is directly attached to an integrated circuit (IC) package will subject the solder columns to shock and vibration impact, as well as long-term compressive load, and therefore should have a light mass to avoid causing excessive damages to solder columns. This limitation becomes a severe problem for large and high power IC packages that need thermal solutions with a high retention load due to heat sink mass or thermal interface requirement. The high retention load often exceeds the maximum long-term compressive load of the solder columns and causes excessive creep, bending, bucking of the solder columns, which finally results in interconnect failures such as shorting or joint failure. Accordingly, the solder columns in a CGA connection often need to be mechanically supported in these applications. The supporting device also needs to be fully fastened, so that the supporting device will not get loose and cause damages by itself. Commonly used supporting devices include posts attached between heat sink and PCB, and external frame or corner support. These devices, however, often require complicated attaching process using epoxy adhesives and consume valuable PCB real estate.